THE BEHAVIOR O F SILVER IODIDE I N T H E PHOTO-VOLTAIC CELL BY ALLEN GARRISON*
Becquerel' was the first to observe that the photo-potential of the silver iodide electrode: electrolyte cell was not always positive. He coated silber with the halide by direct action of the vapors of the halogen and placed the electrode thus formed in an electrolyte. He studied the effect of light on this system by comparing its potential with that of a similar electrode which was kept in the dark. If the silver iodide coating was not too thick on the electrode he found that the photo-potential was always positive, that is the light caused the electrode to take a positive charge from the liquid. But when the coating had reached a critical thickness there appeared a negative light effect which was only temporary and was ordinarily followed by the positive effect. Minchin2 observed similar effects with several cells but regarded the negative effect as exceptional and it has even been stated by more recent investigators that the light effect is always positive in the case of silver electrodes prepared in this way3. In the case of the cuprous oxide photo-voltaic cell it was formerly thought that the photo-potential was always positive but it has recently been demonstrated that the potential induced by the light may be either positive or negative depending on the condition of the electrode and the nature of the ele~trolyte.~Therefore the phenomenon differs from the Hallwachs-Lenard or photo-electric effect in that the direction of the change in potential has not the same significance. It is the purpose of this report to present the experiments which show a remarkable similarity between the action of the silver halides and cuprous oxide. The same general theory applies to both, the conditions which determine both the nature and the direction of the light effects being almost identical. The results are of interest in photography because they furnish new data on the photo-chemical behavior of the silver halides.
Experimental Procedure The light-sensitive electrodes were prepared either by the direct action of the vapors of the halide or by electrochemical deposition of a thin layer of the silver halide on a polished silver electrode. The electrochemical method produced a more uniform coating and the thickness could be controllcd more accurately than by the direct action of the vapors, but the same results can be obtained with electrodes prepared by either method.
* National Research Fellow in Chemistry. Becquerel: La Lumi&re,2, 129(1868).
* Minchin: Phil. Mag. 31,207 (1891). a
Wildermann: Phil. Trans. 206A, 335 (1906). Garrison: J. Phys. Chem. 27, 601 (1g2)3.
3.34
ALLEN GARRISON
The clectrodee were made of thin sheet silver about 3 cm in length and 2 cm in width. They were always coated on the back with paraffin so that only the front surface which was exposed to the light was in electrical contact with the electrolyte. The electrolyte was always saturated with silver iodide. This fixed the product of the silver and iodine ions but their ratio could be set at any desired value by making up the solution with the proper concentration of potassium iodide or silver nitrate as the case demanded. The electrode and electrolyte were placed in a rectangular glass vessel which was painted with black paint on all sides but one. The electrode was fixed facing the transparent side and close to it so that the light had to traverse only 0.5 cm of the elect,rolyte before falling on the electrode. A 0.10N. calomel half-cell completed the photo-voltaic cell. The cell m-as mounted in a light-proof box having a single,rectangular n indow for the admittance of light to the sensitive electrode. The window was covered with a water screen z cm in thickness. T o measure the light intensity a thermopile vas mounted in the box with the photo-voltaic cell as close as possible to the sensitive silver electrode. A moving coil galvanometer was used to measure the current from the thermopile, the deflections being taken as a measure of the light intensity. The e.m.f. of the photo-voltaic cell, that is the potential difference between the silver electrode and the mercury of the calomel half-cell was measured with a high grade potentiometer of standard make. This was the reversible voltage of the cell, since no polarizing current was permitted to pass. A 500 c.p. tungsten filament lamp was used as a light source. When the effect of various intensities of white light was investigated it was necessary to have the same spectral distribution of energy for each intensity since the different colors do not have the same effect. For this reason the intensity could not be varied by changing the temperature of the source and any partial screening method was not desirable. The light source was placed behind a large lense a t a distance from it less than its focal length so that a diverging cone of light was produced. The solid angle of the cone was varied by a slight change in the distance from the lense to the light and thus the intensity of the light over a small area at the center of the cone was varied without any appreciable change in the spectral distribution of energy. Light from the center of this cone was passed through the window of the box containing the photovoltaic cell and thermopilc. Experimental Results T h e conditions which determine the sigit of the photo-potential. There are two factors which determine the sign of the charge which the electrode receives in light. The first is the thickness or density of the silver iodide over the electrode. If all the other conditions such as the light intensity and color and the constitution of the electrolyte remain constant then the photo-potential may be made to be either positive or negative by changing the thickness of the silver iodide coating.
BEHAVIOR O F SILVER IODIDE IN THE PHOTO-VOLTAIC CELL
335
This was demonstrated experimentally by starting with a polished silver electrode and coating it with several successive thin layer8 of silver iodide observing the light effect under fixed conditions after each successive layer of the halide. The results are given in Table I.
TABLE I No. I 2
3 4 5 6
The Influence of the Thickness of the Silver Iodide dE (+> EAAg.+Cal. dE (-) -0.4327 none trace -0.4315 none +0.0015 +o ,003I -0.4296 none +0.0031 trace -0.4275 -0.4273 -0.4270
- 0.0005
-0.0012
+0.0032 +o .0030
E2 -0.4327 -0.4320 -0.4300
-0.4282 -0.4280 -0.4280
The first determination was made with a polished silver electrode in 0.1 normal potassium iodide solution saturated with silver iodide. The electrode was then removed and a thin coating of iodide applied by the electrochemical method. Thus electrode No. 2 had one coating and No. 3 two, etc. Each time the coating was thickened by the additional layer of the halide. I n column I are the original potentials relative to the 0.1N calomel electrode in the dark. I n columns 2 and 3 are the negative and positive photopotentials E (-) and E (+) respectively and in column 4 the potential to which the electrode returned in the dark after illumination. It will be noticed that the positive photo-potential was very small when the electrode had only a small amount of iodide on it and that the effect reached R maximum only after the third coating. Up to this point the light effect was entirely positive. When the fourth layer was applied a slight negalive effect appeared which increased in magnitude with each additional coating. The negative effect was similar t,o that of the cuprous oxide electrode, that is the electrode became negative when the light was first turned on but if illumination was continued at constant intensity the negative charge disappeared and was followed by the positive effect. From the first column it may be seen that the original potential of the electrode in the dark increased with each additional layer of the silver iodide. After six layers the potential had increased 5.7 millivolts. The constitution of the electrolyte had not altered during the measurements, the solution remained saturated with silver iodide and therefore the concentration of the silver and iodine ions was the same for each determination. The change in potential of the electrode must therefore be attributed to some change in the nature of the silver electrode. The direction of the shift in potential would demand that the solution pressure of the silver be decreased, that is the tendency of the silver to ionize decreases as the surface becomes thickly coated with the iodide. This is equivalent to a dilution or decrease in the active concentration of the silver. This has been found to occur in the case of copper also.
ALLEN GARRISON
336
After an electrode had been illuminated it did not return to its original dark potential as may be seen from column 4 but had always shifted toward the original dark potential of electrode No. I, that is -0,4327. This indicates that a partial destruction of the layer of iodide had occurred during the illumination thus increasing the concentration of the silver. This increase in the solution pressure of the silver was accompanied by a decrease in the negative photo-potential. Electrode No. 6 behaved like electrodes Nos. 3 or 4 after the first illumination had partly destroyed the iodide coating and the dark potential increased to -0.4280. This will presently be shown quantitatively. The negative light effects may be made larger and less transient by decreasing the concentration of the potassium iodide in the solution. This brings us t o the second factor which influences the direction of the light effect: the photo-potential may be made to be either positive or negative by varying the ratio of the silver to the iodine ions in the saturated silver iodide solution. The experimental demonstration of this is recorded in Table 11. The same electrode was used in the first eleven observations. It was coated with a moderately thick layer of the halide corresponding approximately to electrode No. 3 in Table I. Electrodes having thicker coatings were also used as indicated in the table. The last corresponded in thickness to No. 6 in Table I. Almost any thickness of coating may be used to observe the effect of various electrolytes but with thin coatings the negative effect is usually only temporary and it is difficult to get large positive effects with very thick coatings on the first exposure to light.
TABLE I1 The Influence of the Ratio of Silver to Iodine Ions T=27’
EA~-+H~ -0.2705 -0 2400 -0.2200 -0.2 I20 0.2050 I
-
- 0.2330 -0.3200 -0.3320 -0.2332 -0.1771 -0.1560 -0.2201 -0.1889
CAl3
+
0.757 x Io-12 2.42
11 11
5.24
7.07
11 1l
CI-
I.42x10-4 .4I2 ’l >l ,190 .I41 l 1
.093
10.7 (2)
1)
dE
- .0030 - .0085 - .or40 - .0160 - ,0262 - .0125
Iodine Ions increased (KI added) ” .316 ” 3.16 l1 8.91 ” .0023 .I12 14.I ” .0032 .Or07 ” (3) Silver Ions increased (AgN03added) 11 .316 l 1 - .0036 3.16 11 .0368 ” - .0124 27.1 61.0 ” .0164” - .OI20 (4) Thicker Coating used and AgNOs added
+ +
5.24
l1
.I90
l1
- .OI95
-0.4270
.0575 l 1 - .0261 (5) Very Thick Coating used and AgNO, added I .86X 10-l~ .os47 -.OOI +.003
-0.3455
4.22
’1
17.4
”
.0023
- ,0260
BEHAVIOR OF SILVER IODIDE IN THE PHOTO-VOLTAIC CELL
'
33 7
The electrode was first placed in dilute Nazi304 solution saturated with AgI. Its potential relative to the 0.1 N calomel electrode in the dark before each exposure to light is recorded in the first column and the photo-potential with its proper sigh, positive or negative, in the fourth column. After each observation a little AgNOB or K I was added to the solution as is indicated thus changing the ratio of the concentrations of the silver and iodide ions. This change can be calculated from the electrode potential. The silver ion concentration in moles per liter corresponding to each electrode potential is recorded in column 2 and the corresponding iodine ion concentration in column 3. The solubility of silver iodide was taken as I.OX IO-^ moles per liter' and the dissociation considered complete a t this dilution. From the first five observationP it may be seen that the negative effect increases with increasing ratio of silver to iodine ions. The positive effect was developed by reversing the process and adding potassium iodide to the solution. This could be repeated as often as desired. Parts 4 and 5 show that a smaller concentration of silver ions is required to develop the positive effect the thicker the coating. This is to be expected from the results of Table I. The reverse is also true, it requires a larger concentration of silver ions to develop the negative effect in the case of electrodes having thin coatings of halide. The same electrode was used for parts I , z and 3 of Table 11; but it will be noticed that the negative photo-potentials in part 3 are smaller than the ones in part I for the same silver ion concentration and even for much larger concentrations. This is another example of the destructive action of the light on the silver iodide coating. This decomposition is more pronounced the larger the concentration of the silver ions and is thus larger the larger the negative effect. T h e photo-chemical decomposition of the silver iodide coating. The halide coating when first formed was yellow white in color. When it was placed in potassium iodide solution and illuminated it remained practically unchanged in color and properties. In silver nitrate solution however where the photopotential was negative the coating was rapidly darkened by the light and its properties changed. Its equilibrium potential in the dark was reduced, that is less positive, and it was found to behave like an electrode having a thinner coating when illuminated a second time. I n case the coating was relatively thin the reversal of light effect took place rapidly and both a negative and a positive effect were observed on illumination, the negative effect appearing first as it did in the cuprous oxide cells. On second illumination the negative effect was usually found to have been destroyed and only the positive effect remained. The decomposition was greatest on electrodes which had large negative photo-potentials and which were in a solution containing a high concentration of silver ions. In the case of electrodes with thick coatings in solutions where Hill: J. Am. Chem. SOC.30, 68 (1908).
338
ALLEN GARRISON
the silver ions were reduced negative effects could be obtained which were more permanent. There was practically no recoil in the positive direction in this case the photo-chemical decomposition being retarded. This was the type of electrode which was used to measure the influence of light intensity on the negative effect. Even under these conditions there was a slight darkening in light and the negative effect was reduced after long exposure. The extent 01 the decomposition for some electrodes is shown in Table 111.
E(dark) - 0.2369
-0,2388 -0.2405
-0.3450 -0.3457 -0.3600
TABLE 111 The Photo-chemical Decomposition of the Coating dE E2 (dark) -0.0065 -0.2415 -0.2436 -0.0055 -0.2430 - 0.0020 Thick Coating and Small C A ~ + -0.0260 -0.3455 Thin Coating and Small C A ~ + +0.0025 -0.3456 +o ,0028 -0.3602
dE2 + o . 0025 +0.0026
-0.0006 -0.0240
+o .0025 +o . 0 0 2 8
The potential in the dark of each electrode is recorded in column I and the effect of the first illumination in column 2. After strong illumination the dark potential was again observed and recorded in the third column and the result of a second illumination in the fourth column. The first three observations were made on electrodes having moderately dense coatings in such a strength of silver ions that the decomposition was rapid. It will be noticed that where the decomposition is most pronounced as indicated by the change in dark potential, the negative effect was greatly reduced. Electrodes which had been darkened in light could be returned to their original condition by the action of the vapors of iodine or by electrochemical coating in potassium iodide solution. T h e relation between the photo-potential and the intensity of the illumination. The measurements of the positive photo-potentials in Table IV were made with a silver electrode covered with a moderately thick film of the iodide and placed in a 0.01normal K I solution. Measurements were made on several different electrodes in order to find the conditions which gave the largest and most uniform positive effects. It has already been pointed out that electrodes having very thin films of the iodide did not have large positive effects. The most satisfactory results were obtained with films as thick as it was possible to use without developing the negative effect. . The relation between the negative effect and the light intensity is shown in Table V. Due to the unstable character of a dense coating of the iodide the results were not as uniform as those for the positive effect, for it was ehown that photo-chemical decomposition is most rapid when the negative effect is largest. The best results were obtained with a very thick film of iodide on the electrode and a comparatively small silver ion concentration. Larger negative potentials were obtained in solutions more concentrated in silver ions but
BEHAVIOR O F SILVER IODIDE IN THE PHOTO-VOLTAIC CELL
339
decomposition was too rapid to compare relative light intensities. Observations were made as quickly as possible to insure the constancy of the electrode.
TABLE IV The Relation between Light Intensity and the Positive Photo-potential EAg+Hg dE I (intensity) -0.3542 .or10 23. I -0.3550 .0095 I8 . o
++
-0.3551
-0.3545 -0.3551
-0.3549 -0.3551 -0.3549 -0.3549 -0.3551 - 0 . 3 549
+ + .0048 ++ .0049 .0040 + .0036 + .0028 + .0024 + .0023 .0082
13.5
4-.0066
9.2 6.o 5.4 4.1 3.5 2.4 2.2
1.75
TABLE V The Relation between Light Intensity and the Negative Photo-potential dE I (intensity) EAg+Hg - .0290 14.0 -0.1887 -0.1896 - .0272 11.9 -0.1886 - .0234 9.5 -0.1886 - .0195 6.5 -0.1887 - .OI80 5.5 -0.1887 - .0134 3.2 -0.1889 - ,0087 1.5 The first column in each table contains the electrode potentials in the dark compared with the 0.1normal calomel cell. The observations were not made in the order of their appearance in the table but those having the smallest dark potentials were made first. During the course of the observations the potential of the electrode in the dark shifted from -0.3554 to -0,3551 in Table IV and from -0.1886 to -0.1896 in Table V. The order of the observations t.0 08 can easily be picked out. The changes in potential on illumination (dE) are recorded in the second column and the light intensity (I) in the ,o third column. The methods of changing and measuring the light intensity have -.OIO already been described. A good idea of the relation between the photo-potential and the light intensity can be gotten from Fig. I. The upper Fig. I curve was plotted with the positive photo-potentials and the lower curve with the negative photo-potentials against the light intensity on the X-axis. While the negative values are larger than the positive ones for the same intensity of illumination the curves d’
-eozo
.
340
ALLEN GARRISON
were found to be of the same form, for when the changes in the pressure of the ions were calculated and the reciprocal of these values plotted against the reciprocal of the light intensities, the points fell along straight lines within the limits of the experimental error. The approximate relation can thus be expressed by the equation 1 n
-
constant
1 -
+
constant ( I ) I for both the positive and the negative effects. The dotted curves are drawn in Figure I to show the behavior of an electrode which was placed in a solution containing such a silver ion pressure that it developed the negative potential first on illumination but the maximum potential was not maintained on constant illumination. The lower curve gives the value of the maximum negative photo-potential for each inteneity and the upper curve gives the potential to which the electrode returned after constant illumination for five minutes. For example when illuminated with light of intensity 15 the electrode first became negative to about -0.01cjo volts but after five minutes constant illumination lost the negative charge and rose to approximately +O.OOIO volts. The cuprous oxide electrodes were found to have these same properties in relation t o the light intensity except that, cuprous oxide electrodes have not yet been made which have the negative effect without the positive recoil as was obtained with silver iodide. Results corresponding to the upper curve and to the two dotted curves have been obtained with cuprous oxide. (Garrison: LOC.cit.) The relation between the color of the light and the photo-potential. While the data on this point are at present limited, enough is known to say that here also there is a marked difference between the photo-voltaic effect and the well known photo-electric effect. There does not appear to be a simple relation between the photo-potential and the frequency but a complicated one depending on the nature of the substance on the electrode. The results in Table VI were obtained by admitting the light into the box containing the photo-voltaic cell and thermopile after passing it through different color filters. The approximate range of wave-lengths admitted by each filter are given in the first column. The light intensity was adjusted to the =
TABLE VI The Influence of the Color of the Light Color Intensity dE Range in microns. Red .81- .65 5 .0008 5 .0008 RY .80 - .53 5 ,0011 RYG .80-.52 .68 - , 5 0 5 .OOI2 OG 4 5 - .49 5 .0007 YG .50--39 2 .0029 GBV
BEHAVIOR O F SILVER IODIDE I N THE PHOTO-VOLTAIC CELL
341
same intensity behind each ray filter (column 2) with the exception of the blue violet where the energy was not sufficient. The positive photo-potential developed in each color was placed in the third column. This is strongly suggestive of the range of sensibility of the silver iodide photographic plate, the effect being greater in the blue and violet with a slight maximum in the yellow green. It is of interest to note in this connection that a silver iodide electrode when dyed in eosine developed larger photo-potentials in the longer wave-lengths just as the photographic plate becomes more sensitive to the red and yellow after a siinilar treatment. A more quantitative investigation of the influence of the optical and chemical sensitizers is being continued. Theoretical Part Since there is such a distinct similarity between the behavior of silver iodide and cuprous oxide in the photo-voltaic cells it is reasonable to conclude that the same general theory may be applied to both substances. It is the purpose of this discussion to show that this may be done and that it furnishes a simple explanation of the photo-chemical properties of silver iodide. The theory which was suggested for the cuprous oxide cells was based on the assumption that the light tends to separate the cuprous ions from the oxygen ions thus increasing the solubility product of the oxide. Silver iodide is soluble in water to the extent of I .oX IO-^ moles per liter. Thus in a saturated solution of silver iodide both silver and iodine ions are present to the extent that their product is 1.0 X 10-l~. If we suppose that the solid halide is more soluble in light then, on illumination, the product of the ions can exceed the value I .oX 10-l~by an amount which depends on the light intensity and color. When both the ions are present in the same amount ( I . I O - ~ moles each), we would expect them to increase in approximately the same proportion in light. When the solution contains potassium iodide such that the iodine ions have a concentration of 0.01moles per liter the concentration of the silver ions are reduced to I.OX1 0 - l ~in the dark. In the light the iodine ions would remain practically fixed in number because of their relatively large amount while the silver ions would increase until the new solubility product was reached. For it requires far less decomposition to double the silver ions for example to 2.0x 1 0 - l ~than to double the iodine ions to 0.01. The reverse would be true if the silver ions had a concentration of 0.01 moles per liter and the iodine ions I.OX10-14 for in this case the iodine ions would be formed in the light and the silver ions remain fixed. A silver electrode whose potential is determined by the reaction Ag l ’ A g f 4- (-1 may be used to measure the increase in silver ion concentration in the solution when illuminated. It would thus become positive on illumination. From the preceding discussion it is evident that this positive photo-potential would increase with increasing potassium iodide concentration.
342
ALLEN GARRISON
On the other hand an iodine electrode capable of the equilibrium I2 e 2 12 (+) may be used in the solution to measure the concentration of the iodine ione. It would become negative on illumination and the maximum changes I- ion concentration would be obtained in a solution containing silver nitrate. The electrode we are considering is the silver electrode coated with a layer of solid silver iodide. It is possible for this electrode to behave either as a reversible silver electrode or as a reversible iodine electrode. Thus the two factors which were found to determine the direction of the photo-potential of this electrode are the factors which determine whether the electrode will behave as a silver electrode or as an iodine electrode and also the extent of the change in the concentration of the ion considered. From the electro-chemical standpoint an electrode can be in equilibrium with an electrolyte only after all the ions in the electrolyte capable of an equilibrium have set up that equilibrium at the electrode. The electrode we are considering when placed in potassium iodide solution cannot be regarded as a silver electrode any more than it ‘can be considered as an iodine electrode, a hydrogen electrode, an oxygen electrode and a potassium electrode. Each substance and the corresponding ion must have pressures determined by the well known expression
+
and therefore we may write
When the electrode is illuminated we suppose thai either or both the silver and iodine ions increase in number so that the problem resolves itself into a determination of the reaction which really determines E.P. and which reaction is determined by the value of E.P. This was considered at some length in the paper on the cuprous oxide cells so that an outline of the conclusions is all that is necessary here. I. At an electrode where several ionic reactions are in equilibrium, a “primary” electro-chemical reaction is one which can be displaced reversibly an appreciable amount a t constant electrode potential, (reactants a t constant pressure). And a “secondary” electro-chemical reaction is one which can be displaced reversibly an appreciable amount only as the electrode potential changes, (pressure of the reactants changes with the action).
It follows that, the “primary” reaction is displaced according to Fara2. day’s law by a current which is passed through the electrode. I n the absence of any “primary” reaction the one which resembles it most closely is displaced but only as the E.P. continually changes.
BEHAVIOR O F SILVER IODIDE I N THE PHOTO-VOLTAIC CELL
3 43
3 . The “primary” reaction fixes the value of the electrode potential by equation (2) and the E.P. thus fixed determines the pressures of all the reactants of the “secondary” reactions by equation (3). In the absence of any “primary” reaction the one which resembles it most closely fixes the value of
E.P.
+
In the case we are considering the reaction Ag*Ag+ (-) is the primary reaction as long as metallic silver and solid silver iodide are present. The reaction may be displaced a t constant pressure since t’he solid silver iodide acts as a reservoir for silver ions. The iodine, hydrogen, oxygen and other reactions are secondary, their pressures being fixed by the primary reaction. When the silver electrode is thickly covered with the halide the silver is no longer constant in amount. Its concentration may be reduced by making the film of halide thicker. Thus it was found that the electrode potential in the dark was more positive the thicker the coating (Table I). In this case the silver ion equilibrium is no longer primary. The silver halide film and the solution about the electrode are able to absorb some iodine. They can thus act as a reservoir for iodine. When the silver equilibrium becomes secondary the iodine equilibrium IBJ 2 I2 (+) resembles a primary reaction most closely and may determine the potential of the electrode. From this consideration the positive photo-potential is to be expected when the coating is relatively thin and the iodine ion concentration relatively large. On the other hand the negative photo-potential is to be expected when the silver is cut off from the solution by a thick layer of the iodide and the silver ion concentration relatively large. T h e photo-chemical decomposition of the halide may be traced to an electrochemical deposition of metallic silver. The decomposition was found t o be greatest when the largest negative photo-potentials were developed in the presence of a high concentration of silver ions. It is obvious that these are just the conditions under which the silver ions would have the greatest tendency to deposit. It has been known for a long time that silver iodide emulsion on the photographic plate is not sensitive to light when precipitated in an excess of potassium iodide solution. The plate may be sensitized by dipping in silver nitrate solution. While silver nitrate forms complexes with iodine the theory that this alone accounts for the sensitizing action of silver nitrate does not take into account the fact that potassium iodide also forms complexes with iodine and should also sensitize the plate, rather than retard its action. If the pho to-chemical decomposition is an electrochemical deposition as is suggested here, each particle of halide in the emulsion may be regarded as an electrode and it would be less stable in light when the conditions were such that it developed a negative photo-potential in the presence of excess silver ions. The conditions would be fulfilled by dipping the photographic plate in silver nitrate solution.
+
344
ALLEN GARRISON
This theory may be harmonized with either the metallic particle theory or the solid solution theory of the latent image, both theories have already been suggested.' The silver may be deposited over the exposed area as very fine particles of metal or dissolved in the halide in the form of a solid solution. When a layer of silver halide is thus decomposed by light the particles of silver give the electrode a dark color. After this has happened the reaction Ag AgS (-) becomes primary once more and the electrode loses its negative photo-potential. This would also be accompanied by an increase in the negative electrode potential in the dark. This accounts for the changes represented in Table 111. The reEation between light intensity and the photo-potential can be obtained from the Einstein photo-chemical law in the following way: Since the number of quanta of any color of light is proportional to the intensity of radiation of that color, then the amount of decomposition may be put proportional to the intensity of illumination so long as all the active colors are changed in the same proportion. The reaction we are considering is a first order reaction so that the amount of decomposition may be proportional to the amount of silver iodide also. Let N o= the amount of AgI present in the dark. and n = the amount of Ag+ formed by the light. and N = N o - n = the amount of AgI left a t intensity I. n =constant . N I
+
N,I
=-
-nI k
-- No1 - (k + I) The values of No, N and n may be expressed in terms of concentration or moles per unit volume. then d E
=
RT n+no - ln() vF no
+ -)non
or d E = C log
(I
Oherefore d E
C log
=
where no = PAg in dark
let No' =
-
NO
no
___
[(kYI)
+
ll
(C) is a constant depending on the temperature and the valence of the ion. k is a constant depending on the nature of the substance and the illumination. It may also depend on the magnitude of No'.When k is small compared with Sheppard and Wightman: Science 58, 89 (1923).
BEHAVIOR O F SILVER IODIDE IN THE PHOTO-VOLTAIC CELL
345
I, that is for very large values of I, d E approaches a constant. For very small values of I, d E approaches 3C log (const * I+1) and n becomes linear with I. 1 1 The general relation is expressed in equation ( I ) , that is -is linear with - * n I The experimental results are not accurate enough over a wide range to test the theory closely and get the exact values of No and IC, but it is interesting to note that the approximate relation can be obtained from the simple theory. The same calculation can be applied to the negative photo-potential where (n) represents the amount of iodine ions liberated in the light and (no) the amount in the dark at equilibrium. Goldmannl has suggested a theory for the photo-voltaic cell based upon the Hallwachs-Lenard effect or photo-emission of electrons and Sheppard and Wightman (loc. cit.) advanced a similar theory for the photographic plate. If this idea is accepted it is difficult to see how the negative photo-potential can be formed under the conditions described above and bear the same relation to the light intensity as the positive effect. This would require that the solid particle of silver or halide develop a negative charge by the emission of a negative electron. The phenomena may be explained however by the theory that the primary effect of the light is a separation of the charged elements as ions. Summary Silver iodide photo-voltaic cells have been made which have both positive and negative photo-potentials. The sign of the photo-potential was found to depend on the thickness of the silver iodide layer over the electrode and on the ratio of the concentration of silver ions to that of iodine ions in the electrolyte. The relation between the light intensity and the changes in electrode potential were measured for both the positive and the negative effects. Decomposition of the silver halide was found to occur during the maximum negative effects in the presence of a high concentration of silver ions. The results are in agreement with the theory that the silver iodide becomes more soluble in light. The relation between light intensity and photopotential has been obtained with the aid of this theory. The Rice Institute Houston, Texas.
-
-_____
* Goldmann: Ann. Physik. (4) 44, gor
(19x4).
